In order to make the LTE-Advanced reach 1 Gbps of downlink rate and 500 Mbps of uplink rate and also make full use of various scattered frequency bands, the carrier aggregation (CA) has been introduced in 3GPP Release 10 (Rel-10). By aggregating at most five 20 MHz component carriers (CC) to allow the system bandwidth to reach 100 MHz, the requirements of rate are satisfied. The cell of a primary component carrier (PCC) is a primary cell (Pcell), and the cell of a secondary component carrier (SCC) is a secondary cell (Scells). All Scells are added, modified and removed via RRC reconfiguration messages by a serving eNodeB of UE, where an RRC information element added with a secondary cell, as shown below, includes secondary cell index (ScellIndex), physical cell identifier of the secondary cell and downlink carrier frequency:
SCellToAddMod-r10 ::=SEQUENCE { sCellIndex-r10 SCellIndex-r10, cellIdentification-r10 SEQUENCE {physCellId-r10PhysCellId,dl-CarrierFreq-r10ARFCN-ValueEUTRA }OPTIONAL, --Cond SCellAdd
Compared with Release 8 (Rel-8), the introduction of the multi-carrier aggregation also brings about larger power consumption of user equipment (UE). To further reduce the power consumption of UE, in Rel-10, other than the previous discontinuous reception (DRX), quick activation and deactivation solutions for secondary cells are also introduced.
When a secondary cell is deactivated, UE does not monitor the physical downlink control channel (PDCCH) information of the cell and also does not transmit data in this cell. Meanwhile, the UE does not perform for this cell measurement of channel state information (CSI) (the measurement of a downlink common reference signal still needs to be performed).
Both activation and deactivation are controlled by an eNodeB. As shown in FIG. 1, a schematic diagram of the format of an MAC CE for quick activation and deactivation of a secondary cell is shown. By sending an 8-bit MAC control element (MAC CE), an eNodeB controls the activation and deactivation of one or more SCells, where Ci indicates whether a serving cell i is to be activated; if Ci is set to be 1, it is indicated that the serving cell i needs to be activated, and if Ci is set to be 0, it is indicated that the serving cell i needs to be deactivated; and, R is a reserved bit. If no PDCCH message or data is received from an activated secondary carrier within a time period set by a deactivation timer, the UE may deactivate a certain SCell actively. Of course, the eNodeB may configure the deactivation timer as “infinite” to prevent the UE from deactivating a certain SCell actively.
In 3GPP Release 12 (Rel-12), the requirements of small cell enhancement (SCE) are proposed. Target scenarios for small cell enhancement include scenarios covered by macrocells, scenarios not covered by macrocells, indoor scenarios, outdoor scenarios, and enhanced scenarios with ideal and non-ideal backhaul, as shown in FIG. 2.
A macro eNodeB serves as the master eNodeB of UE, while a small cell eNodeB serves as the secondary eNodeB of the UE. The macrocells and the small cells may work on different frequency bands. In a case of the coverage of a macrocell, inter-eNodeB carrier aggregation (Inter-eNodeB CA) may be employed, where the primary component carrier (PCC) can be located in the master eNodeB only. According to the consensuses made in the 3GPP meeting, both the master eNodeB and the secondary eNodeB have the capability of independent scheduling and the capability of independent physical uplink control channel (PUCCH) transmission, and both the master eNodeB and the secondary eNodeB can aggregate more than one component carrier, i.e., intra-eNodeB carrier aggregation compatible in Rel-10. All cells serving the UE under each eNodeB are called a cell group (CG), all cells serving the UE under a master eNodeB are called a master cell group (MCG), while all cells serving the UE under a secondary eNodeB are called a secondary cell group (SCG).
Through the discussions in the study item (SI) phase, the inter-eNodeB carrier aggregation inherits the characteristics of the intra-eNodeB carrier aggregation. Each UE may have at most five serving cells, i.e., five component carriers. The addition and removal of all component carriers are executed by the master eNodeB. For the activation and deactivation of a secondary cell, the following consensuses or agreements have been reached in the SI phase: 1) cells in an SCG support activation and deactivation; 2) the master eNodeB takes charge of the activation and deactivation of cells in an MCG, while the secondary eNodeB takes charge of the activation and deactivation of cells in an SCG; and 3) an SCell added firstly in an SCG has the capability of PUCCH transmission, and this cell is always in the activated state.